Residual current devices (RCDs) are widely used for the detection of differential or residual currents flowing in AC and DC systems. These will be familiar to those versed in the art, and examples of such devices are given in International Application No. PCT/EP2011/066450, which is incorporated herein by reference in its entirety.
RCDs tend to operate at current levels starting at several milliamperes, but typically not less than 5 mA. However leakage current detectors are generally required to operate at lower current levels, making the use of RCDs impractical for this application.
The principle of operation of leakage current detectors is well known to those versed in the art, and an example of such a device is given in U.S. Pat. No. 4,931,894, the disclosure of which is incorporated herein by reference in its entirety. Whilst the detection of leakage currents on AC systems is relatively simple, this is not the case for DC systems, especially a floating DC system as found in photovoltaic systems and electric vehicles.